Problem with Sims not aging

[Aly2578]

I actually like to stop my sim from aging.  I don't have enough time to develope a relationship into something more than best friends before I'm ready to retire.  I tried going into the menu and then options, but it is greyed out and I can't click on it. Does anyone know anyother way to disable it so I can play longer with one sim without having to create a new one?  I have just Sims 3 nothing added at all.
Any help would be appreciated.

Thank You